Container filter boxes are commonly used to separate solid particles from a mixture of solids and liquid (usually water) for disposal. Examples of applications for such filter boxes include filtering wet contaminated soil from a construction site where the liquid is drained from the sludge prior to disposal and filtering sediment taken from the bottoms of refinery storage tanks which is generally a heavily liquid-laden material. Typically, container filter boxes comprise an outer liquid-impermeable rectangular shell having an open upper end and an inner filtering shell having an open upper end and perforated bottom, side and end walls. The inner shell is supported within and spaced from the outer shell by a support frame so as to define a material collection area between the inner and outer shells. The inner shell is lined with a disposable filter cloth of a desired mesh size. These filter boxes are loaded with sludge or other liquid-laden material through their open upper end, whereupon liquid and smaller solid particles pass through the filter cloth liner and the perforated inner shell into the spacing between the inner and outer shells where they are removed through drains provided on the bottom of the outer shell. The inner and outer shells also define a large hinged door at one end of the box to provide access to the interior of the filter box for removal of the larger solid material retained on the inner shell. However, some of the finer solids which pass through the perforated inner shell of the filter box do not pass through the drains and become trapped between the inner and outer shells. Solid material also falls into the area between the walls of the two shells during the loading of the filter box which can both contaminate the liquid collected between the bottom walls of the shell and add to the volume of trapped solid material. Removal of the trapped material is attempted by directing water under pressure through the perforations in the inner shell with the hinged end door in the open position in an effort to flush the trapped solid particles out from between the shells through the open ends of the two shells. Users of these container boxes, however, have experienced substantial difficulty in obtaining a thorough cleaning of these boxes following use. The filter box of the present invention both obviates the cleaning problem and prevents solids from passing between the walls of the box during loading.